The present invention relates to mixing elements and methods and, more particularly, to an assembly of crossing elements such as found in static mixers and heat exchangers and to a method of constructing same.
Static mixing elements are positioned in tubes or other fluid flow conduits to cause mixing of one or more fluid stream flowing within the conduit or to cause simultaneous mixing of a product fluid stream and heat exchange between the product fluid stream and a service fluid separated from the product fluid stream by a wall and flowing in co-current or countercurrent relationship. The fluid streams include polymer melts and other highly viscous fluids in laminar flow and low viscosity liquids or gases in turbulent flow applications. These static mixing elements typically have no moving parts and operate by radial transport of the fluid stream and dividing the fluid stream into multiple partial streams which are then recombined to reduce cross sectional variations in composition, temperature or other properties of the fluid stream. In types of static mixing elements generally known as SMX, SMXL, SMV and SMR mixers, two or more grids of crossing elements are arranged at intersecting angles to each other and at an angle to the longitudinal axis of the conduit. The crossing elements, which are corrugated plates in the case of SMV mixers, bars in the case of SMX and SMXL mixers, and rods or tubes in the case of SMR mixers, are spaced apart within each grid and crossing elements from the paired grid are interposed within the spacing. In order to achieve good mixing, the crossing elements are normally placed closely together so that there is no, or only a little, gap between adjacent elements.
Static mixers as described above are often used for enhancing the heat transfer between a service fluid and a product fluid stream separated from the service fluid by a conduit wall. In the case of SMV, SMX and SMXL type mixers, the crossing elements are inserted in a jacketed pipe or inside the tubes of a multi-tube heat exchanger. The service fluid then flows outside of jacket or shell and the mixing and heat transfer with the product fluid stream flowing within the pipe or tubes is enhanced by the crossing elements. In the case of SMR mixers, the bars in the crossing elements are replaced by tubes arranged in multiple, parallel tube grid. The service fluid flows within the tubes and the product fluid stream flows outside the tubes and is mixed while simultaneously undergoing heat exchange with the service fluid.
One problem with static mixers using grids of crossing elements of the types described above is the difficulty in making them strong enough to withstand the pressure drop caused by viscous fluids, such as polymers, flowing through the mixers. The crossing elements must also be secured to the flow conduit and those crossing elements secured to the conduit must withstand the stresses applied to the other crossing elements. In many applications, such as fiber coolers, the SMR tubes must additionally withstand a high outside pressure.
In order to withstand these stresses, the crossing elements must have a rugged design involving very thick materials and reinforcing components, such as welding the crossing elements together at their crossing points. In the case of SMR type mixers, it is known to additionally weld tabs between each adjacent loop of tubing within each tube array. The tabs are normally the same thickness as the tube wall and up to three rows of tabs are placed in each tube array. A typical SMR tube bundle may consist of eight to more than forty such tube arrays and, as a result, more than two thousand tabs might be required for a typical SMR tube bundle. It can be appreciated that welding or otherwise securing these tabs to the tubes is extremely labor-intensive and can add considerably to the cost of the tube bundle.
A significant need has thus developed for an improved method of reinforcing the above-described crossing elements.